The present application is based on Japanese Patent Application No. 2001-068535, which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a light pipe which can be used for a front light unit to form a reflective liquid-crystal display device free from disorder and moirxc3xa9 of a display image, bright and easy to view, and relates to a planar light source unit using the light pipe and excellent in effective light-utilizing efficiency.
2. Description of the Related Art
In the related art, there has been known a front light type reflective LCD (liquid-crystal display device) operative in a reflection-illumination double mode. In the reflective LCD, a side type light pipe having light output means constituted by prismatic structures shaped like stripes and formed in an upper surface of the light pipe at equal intervals is disposed on a visual side of a reflective liquid-crystal display panel, so that display can be viewed even in dark places. In the front light type reflective LCD, light incident on a side surface of the light pipe is made to exit from a back side (lower surface side) of the light pipe, and is reversed by use of a reflection layer. In such a manner, display light is viewed through the light pipe. Accordingly, the front light system has an advantage in that a total reflection type reflection layer can be used to enhance reflectance and in that a display image obtained by the reflective LCD can be therefore made brighter both in a refection mode and an illumination mode than that obtained by a semi-transmissive LCD using a backlight unit and a half-silvered mirror.
On the other hand, the light pipe for a front light unit has the following advantages in terms of viewing characteristic. First, luminance is excellent because light is made to exit from the lower surface of the light pipe efficiently with good vertical directivity. Second, light leakage from the upper surface of the light pipe is little though such light leakage disturbs viewing of a display image. Third, the light pipe used does not make the display image disordered. Fourth, the light pipe transmits external light efficiently. Fifth, existence of the light pipe is hardly conspicuous visually. The related-art light pipe, however, has the following problems. That is, viewing of a display image is disturbed because the striped prism structure is apt to be conspicuous visually and existence of the light pipe is accordingly easily noticed. In addition, display quality is largely lowered because the striped prism structure interferes with pixels of the liquid-crystal display panel and moirxc3xa9 is therefore generated. As a measure to prevent such moirxc3xa9, there has been proposed a system in which the direction of arrangement of prisms is inclined to the direction of arrangement of pixels. In such a case, however, the effect in preventing moirxc3xa9 is poor if the inclination angle is too low. If the inclination angle is conversely too high, light transmitted through the light pipe is reflected horizontally on faces of the prisms. Accordingly, there arises also a problem that the exit angle is largely inclined to reduce illuminating efficiency and output efficiency.
On the other hand, a structure of spherical dots or sectionally cylindrical concave/convex pits has been known as the light output means provided in the light pipe. Light reflection using the spherical dots or sectionally cylindrical pits is, however, ineffective for illumination of a reflective LCD because light is diffused and made to exit from the light pipe at a high inclination angle to a direction normal to the lower surface of the light pipe to make vertical directivity poor. That is, because the reflection surface of the reflective LCD generally takes a normal distribution type reflection mode, reflectance increases as light is reflected more regularly on the reflection surface. Accordingly, if light made to exit from the lower surface of the light pipe is short of vertical directivity, the reflected light through the reflection surface can hardly illuminate the LCD effectively even in the case where efficiency in light emission from the lower surface is excellent. As a result, the reflected light hardly contributes to improvement in luminance of the LCD, so that display of the LCD is dark in the frontal direction. Generally, the direction that an observer views the LCD is a direction normal to the LCD, that is, a direction normal to the lower surface of the light pipe. Accordingly, light made to exit in the direction normal to the lower surface of the light pipe can illuminate the LCD most effectively.
Further, in the light pipe having a structure of semispherical dots or pits as described above, portions having inclination angles close to those of flat portions are formed continuously. In reflection of external light, light reflected through these portions having inclination angles close to those of the flat portions is apt to be caught into the observer""s eyes as if the light were continued to light reflected through the flat portions. As a result, there arises a problem that the dots or pits are apt to be conspicuous visually and that existence of the light pipe is accordingly apt to be recognized visually.
An object of the invention is to develop a light pipe for a front light unit, which is visually inconspicuous both in an external light mode and in an illumination mode, substantially free from generation of moirxc3xa9, and formed so that light incident on a side surface of the light pipe is made to exit from a lower surface of the light pipe efficiently with good vertical directivity to thereby make it possible to form a reflective LCD being bright in display and providing a display image hardly disordered.
According to the invention, there is provided a light pipe having a plate-like substance having an upper surface, a lower surface opposite to the upper surface, an incidence side surface, and an end surface opposite to the incidence side surface, the plate-like substance including light output means formed in the upper surface so that light incident on the incidence side surface is made to exit from the lower surface through the light output means, the light output means being constituted by fine grooves arranged discontinuously so as to face the incidence side surface, each of the fine grooves having a length of not larger than 250 xcexcm and a depth of not larger than 50 xcexcm and not larger than ⅕ as large as the length, each of the fine grooves being constituted by a combination of an optical path changing slope inclined to the lower surface at an angle in a range of from 35 to 48 degrees and a steep slope facing the optical path changing slope and inclined to the lower surface at an angle of not lower than 60 degrees.
According to the invention, there is provided a planar light source unit having a light pipe defined above, and a light source disposed on the incidence side surface of the light pipe.
According to the invention, there is provided a reflective liquid-crystal display device having a light pipe defined above, and a reflective liquid-crystal display panel disposed on the lower surface side of the light pipe.
According to the invention, there is provided a reflective liquid-crystal display device having a planar light source unit defined above, and a reflective liquid-crystal display panel disposed on the lower surface side of the light pipe in the planar light source unit.
According to the invention, it is possible to obtain a light pipe in which light incident on an incidence side surface of the light pipe is made to exit from a lower surface of the light pipe through optical path changing slopes efficiently with good vertical directivity. The light pipe can be used for obtaining a planar light source unit excellent in light-utilizing efficiency and excellent in frontal luminance. Because the light pipe includes light output means constituted by fine grooves arranged discontinuously, moirxc3xa9 owing to interference of the light output means with pixels arranged in a liquid-crystal display panel is hardly generated and existence of the light output means is visually inconspicuous both in an external light mode and in an illumination mode. In addition, the light pipe can transmit external light efficiently because light leakage from an upper surface of the light pipe is little. Accordingly, the light pipe can be provided as a front light type light pipe for forming a reflective LCD bright in display and providing a display image hardly disordered.
Features and advantages of the invention will be evident from the following detailed description of the preferred embodiments described in conjunction with the attached drawings.